Engine hesitation problem

1950 - 1952, M38, questions, discussions, regarding anything related to the M38.
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Scott in NM
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Engine hesitation problem

Post by Scott in NM » Sun May 23, 2021 3:29 pm

After a six week hiatus, finally got my engine tuned. Purrs like a kitten at idle, pulls 17" Hg at 7000' with a steady needle, so quite satisfied so far. HOWEVER,

A little tough to start, requires fiddling with the choke but after 5-6 seconds it fires off with a puff or two of black smoke. Unfortunately, there is no power when accelerating. Once I nurse it to about 10 mph in first gear it takes off like a rocket. Shift into second around 20 mph, bogs down, catches up around 25, and then takes off like a rocket. Haven't made it into third yet but expect similar results.

Black smoke indicates too much fuel, so leaned the mixture as much as possible (about 3/4 turn out from full in on the screw) to maintain smooth idle with the above results . Both fuel pump and carb professionally rebuilt (yes, I checked the float level in the carb). Do I not have a vacuum advance? What can I check? Any inputs greatly appreciated and I'll test/alter/adjust per suggestions.

Scott


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Re: Engine hesitation problem

Post by mdainsd » Sun May 23, 2021 3:42 pm

The mixture screw only affects the idle circuit. Once you come off idle it is not in the equation anymore.

First things first: Original style carb, or something else?

Who rebuilt the carb? If it is an original M38 carb there are a couple of pitfalls rebuilding them. One is having the springs under the diaphragms switched. The other is a lot of the rebuild kits come with one of the parts that goes into the power jet (I am going from memory) for both the M38 and the M38A1, they can't be interchanged or you get running problems. 5 seconds of cranking to get it started seems long. Mine fires with a touch of the starter, hot or cold. Frankly if it was to turn over more than a couple of revolutions I would start looking for where the trouble was. Cold start for mine is throttle out one notch, choke out no more than 1/2". No foot on the gas pedal, hit the button and she's running and idling. Hot, is just touch the button again not touching the gas pedal.
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Scott in NM
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Re: Engine hesitation problem

Post by Scott in NM » Sun May 23, 2021 7:07 pm

Sorry, missed a few details.

Entire jeep is as it came from the factory complete with underwater ventilation system, so all original parts.

Prior to rebuild it would start by just tapping the starter with half choke and it would fire up within half an engine RPM just like yours, so I know what it can do. As far as the rebuild, we all know the rebuilder and his reputation is without question. So let's assume everything is all good but just needs tweaking.

Vacuum advance? Vacuum leak? Gremlins? I know someone out there has the magic key, where you hiding?

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Re: Engine hesitation problem

Post by Scoutpilot » Mon May 24, 2021 2:05 am

Have you tried it with the fording system tube disconnected from the carb?
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Scott in NM
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Re: Engine hesitation problem

Post by Scott in NM » Mon May 24, 2021 5:03 am

I've had the crossover tube to carburetor tube removed and installed. I've not tried removing the copper tube.

I know this has centrifugal advance. I had to replace the bushings on the shaft so maybe I screwed something up there. I can move the distributor rotor and it stays where I place it. I think if the advance were working it should spring back to the original position. Can anyone confirm/refute this?

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Re: Engine hesitation problem

Post by mdainsd » Mon May 24, 2021 5:24 am

Yes. With a mechanical advance distributor you can grab the rotor and twist it a small amount and yes it should spring back.
52 M38 2X, '52 M37, '44 WC51, '42 WC 56/57, '50 CJ-V35(U), '42 GPW, '44 M3A1 Scout Car, 2010 M-Gator

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Re: Engine hesitation problem

Post by Scott in NM » Wed May 26, 2021 11:02 am

I can now start the jeep and it hums, not purrs, at idle it is so smooth, so this is a start. However, still have a chugging problem. If I am pointed downhill and get rolling, it has plenty of power and goes like a rocket. Unfortunately, going up a hill, or (god forbid) trying to shift and there is a load, it chugs. If in first gear, I must use about half clutch to move forward and gain speed to about 10 mph, then it "catches" and I'm off like a bat out of hell. Shift into second at anything but a very high rpm and it will bog down. Once it "catches", I'm off again.

The distributor has been disassembled to check the mechanical advance (all appears good here). Distributor is rotated full clockwise now and engine now pulls 17 mm Hg at idle (and 7000'). In attempting to connect the two ventilation tubes, they suggest that the distributor should be retarded (CCW) to about half position where it was before the rebuild and started and ran like a charm. mdainsd, since yours runs like mine used to, could you send me a picture of your distributor position (text to 505 280 6417 or email to kns1313@q.com) so I can see what really good looks like? I'd also love to hear from others with the original 24V distributor to get a feel where normal should be.

Do I need a little more advance on the timing to eliminate the chugging? I'spent two days a month ago adjusting the position of the oil pump splines, now that I'm back and soooo close to running well I'm willing to try anything. Back then adjusting by one spline was too much but now . . ??? There are 12 splines on the oil pump shaft so 30 degrees per spline. If I rotate the oil pump shaft CW one spline (looking at it from the left side of the engine) and reinsert it, is there enough travel rotating the distributor that I can still make adjustments after I start from the fully retarded (full CCW) position?

So, is the lack of power at low rpm a timing or advance problem? I've eliminated carb, fuel pump, and bad gas. Love to hear from any of you M38 mechanical brainiacs.

Scott

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Re: Engine hesitation problem

Post by W. Winget » Wed May 26, 2021 1:00 pm

Usually when I adjust timing, I go for max RPM then back down slightly. Punch the throttle a few times to ensure it's not too far advanced and backfiring or hesitating, then try to drive it, and see how it turns over, if either are off (can't get speed, or really hard to crank) I will adjust the distributor a little more.
Have you tried just loosening it then starting and adjusting?
Timing lights are great, but many things get changed over 75+ years of engine life.
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PS you have triple checked your wires to ensure two are not backward according to your distributor rotation and firing order. waw
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Scott in NM
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Re: Engine hesitation problem

Post by Scott in NM » Wed May 26, 2021 3:04 pm

I'm at full advance travel, ain't no more to move. If I want to do this, then I would have to move the oil pump one spline. Last time I did that I couldn't get it started as now I was too advanced even though the distributor was up against the retarded stop. So the question is: Now that it is five weeks later since I've last done this, does it make sense to do it again given the discussion above? (I have thoughts of Einstein's definition of stupidity.)

Oh, were it to be so simple to just change the spark plug wires. Yes, placed per the manual. But a valid question!

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Re: Engine hesitation problem

Post by mdainsd » Wed May 26, 2021 3:44 pm

Scott, Mine is currently sporting a civilian distributor, so I don't know if that does you any good?
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Re: Engine hesitation problem

Post by W. Winget » Wed May 26, 2021 3:51 pm

And rotor turns the same way as that manual?
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Scott in NM
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Re: Engine hesitation problem

Post by Scott in NM » Wed May 26, 2021 5:57 pm

mdainsd - No, I wouldn't think a civvie model will work.

Winget - Yes, turns as normal, 1-3-4-2 CCW

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Re: Engine hesitation problem

Post by dpcd67 » Thu May 27, 2021 6:47 am

Yes a civilian distributor will definitely work on an M38 as long as you also have civilian wires. And a coil on the block; the studs are already there for you.
Yes you can move the wires on your distributor to reclock the timing, if you are at the end of the advance or retard cycle. Probably should move the oil pump though.
Rotors turn counterclockwise as you look down on them. Military sealed or civilian open; same thing inside.
If you lose your place, then go back to TDC on #1 cylinder and start over. Put your thumb on the spark plug hole and turn the engine with a socket on the crank nut. You can feel when you get to TDC. Ok, you do need another person to help. Not your wife. They don't like doing this.
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Scott in NM
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Re: Engine hesitation problem

Post by Scott in NM » Thu May 27, 2021 4:19 pm

I've probably done all this at least two dozen times. To add insult to injury, last night I burned out the coil. Replaced it and in so doing stripped both mounting holes holding the new coil. Buttoned everything back up and now the g*****n wire going to the distributor instantaneously smokes as soon as the ignition switch is turned on. Plus the gauges don't move. So now I've screwed up something else. After many days of just screwing with it, I'm tempted to just push it out into the driveway and set it on fire.

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Re: Engine hesitation problem

Post by sjalbert » Tue Jun 01, 2021 7:41 am

Take a deep breath, maybe take a break for a couple of days. Mine died on a drive yesterday. Turned out to be a loose wire at the coil, the ring connector had come loose.
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